Mechanical Design, Construction and Alignment of the ISAC RFQ ...

dinal growth of 2.0 mm. The elasticity of the mounting
system copes with this and growth returns to zero at operating
temperature.
3 ACCELERATOR COMPONENTS
3.1 Design Considerations
Each ring is identical and consists of a base, a stem, a split
strong back, two electrode supports and an rf skin (Fig. 1).
To achieve thermal stability all rf surfaces are water cooled
(except for the outer rf skin). This is done by using two separate
water circuits. Circuit 1 supplies cooling water from
the inlet manifold to a tube array soldered on the inside of
the inner rf skin; from there the water enters a circuit drilled
into the electrode support (Fig. 2) and then returns to the
outer manifold. Circuit 2 transports water from the inlet
manifold to the electrode supports through the electrodes to
the adjacent ring. Hence, with a 3-ring platen, number 2ring
supplies water to the 4 electrodes, and number 1 and 3
rings receive and return the water. The rf skin encloses the
strong back but is attached only where it meets the electrode
supports, thus allowing for thermal growth of the skin without
loading the structure and causing electrode misalignment.
Prototype tests indicate a very small movement during
voltage ramp-up but this returns to zero at steady state.
Tests on the seven rings showed no movement at all.
As mentioned the philosophy of mechanical positioning,
or alignment of the rings, in order to control the accuracy of
the electrodes around a true datum line in space (i.e., beam
centerline), is to make each ring identical such that when
sitting in a jig on its baseplate the electrode mounting pads
are within 25 m, of true position. In the tank there are 6
platens – the first supports 4 rings, and the remaining five 3
rings each. Each platen consists of a special steel 63.5 mm
thick with an offset longitudinal rail bolted and doweled in
place. The platen and rail are accurately ground in one set
up, thus providing accurate datums for mounting and locating
the ring bases (the flatness tolerances, i.e., 12 m).
The platens have 5 adjustable mounting points - 3 vertical
Figure 2: Electrode support showing holes drilled for water
cooling circuits.
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and 2 lateral. Each platen is adjusted in the tank using special
targets for each platen and aligned by the theodolite
intersection procedure which is covered in a later section.
Once the platens are aligned they are locked in position and
are ready for installation of the rings.
3.2 Fabrication Considerations
In order to achieve the manufacture of identical rings a
philosophy of fabrication and assembly had to be decided
upon. The key to this is that the mounting surface on which
the vane shaped electrodes are mounted are machined as the
last major operation. In order to achieve this with such an
awkward shape it was decided to use Electrical Discharge
Machining (EDM) and employ a master fixture that would
engage datum features on the ring base (the same features
that would eventually engage the datums on the platen) and
accurately hold the ring in the EDM machine for final machining,
thus ensuring that all rings are machined in an identical
set up. This master fixture also had provision for addition
of inspection tooling to check the result. The same
fixture was also used to manufacture special targets previously
mentioned which are used to align the platens. Finally,
each ring is inspected by an independent organization
using a coordinate measuring system. This method of ring
manufacture means that individual components can be held
to reasonable tolerances with enough machine allowance on
the electrode platform to accommodate the accumulated assembly
tolerances. Prior to EDM machining at final assembly,
all components are doweled together and cleaned.
The electrode support is also of some complexity due to
the cooling circuits drilled within the shape of this component
(see Fig. 2). This is a fully NC machined component
produced to a better than 0.80 m surface finish in
chromium copper (tellurium copper was chosen due to a
higher machinability and conductivity rating but pieces of
this size are unavailable).
The only other components that require special mention
are the electrodes and the rf skin. The electrodes are
13 mm x 38 mm x 120 cm long (except the first two sets
which are 80 cm long) and made from tellurium copper for
improved machinability and good conductivity. The rods
are rough machined, gun drilled for water cooling channels
and straightened by the supplier. The cooling holes are
plugged at either end with silver soldered plugs. They are
then finally machined and profile cut on an NC machine to
an overall tolerance of 25 m. Both the electrode and
electrode support quality and accuracy met or exceeded our
expectations.
The rf skin is made from 2.2 mm thick C110 copper and
the u-shaped ring was created by spinning over a mandrel
in two pieces that were later brazed together. Mounting
frames and flanges are soldered and finished machined and
the cooling array is then soldered in position.